1. Field of the Invention
The present invention plates to a projection lens for a microlithographic projection exposure apparatus. Such apparatuses are used for manufacturing microstructured devices such as integrated circuits.
2. Description of Related Art
In numerous optical systems, a high imaging quality requires that the light passing through the optical system is in a defined polarization state over the entire beam cross section. Since said defined polarization state does not necessarily have to be constant over the beam cross section, this requirement is also referred to as “defined polarization distribution”. If disturbances of said defined polarization distribution occur, it can result in unacceptable imaging errors and/or in contrast losses in the image plane of the optical system. Such disturbances may be caused, for example, by polarization-dependent reflecting layers or birefringent lens materials.
This issue has become particularly important in connection with microlithographic projection exposure apparatuses, such as those that are used, for instance, to produce large-scale-integrated electrical circuits. In the projection lenses of such apparatuses lens elements composed of fluorite (CaF2) crystals are increasingly being used since this material is still highly transparent even if the projection light has a wavelength in the deep ultraviolet (DUV) spectral range. However, fluorite crystals are naturally (i.e. intrinsically) birefringent at such short wavelengths; in addition, a so-called stress-induced birefringence that is caused by mechanical stresses in the crystal lattice may occur.
If it is not possible to suppress the stress-induced birefringence by suitable measures and to achieve compensation for the retardance caused by intrinsic birefringence, the disturbances in the polarization distribution caused by these effects have the result that aberrations occur in the desired intensity distribution downstream of a polarizing optical element. Here, optical elements are referred to as “polarizing” whose reflectance and/or transmittance depends on the polarization direction of the light.
If, for example, the object plane of a projection lens of a microlithographic projection exposure apparatus is uniformly illuminated and a disturbance of the polarization distribution occurs, undesirable local intensity fluctuations occur downstream of a polarization-selective beam-splitter layer that is transparent to light having undisturbed polarization. This is due to the fact that those light components having a disturbed polarization cannot pass through the beam-splitting layer. The intensity fluctuations may result in a non-uniform illumination and, in particular, in fluctuations in the line widths of a photosensitive layer to be exposed in the image plane of the projection lens. Such fluctuations in the line widths reduce the clock frequencies of the large-scale-integrated electrical circuits and are therefore undesirable.
For the reasons mentioned, attempts are being made to avoid the occurrence of disturbances in the polarization distribution from the outset. To compensate for the delays caused by intrinsic birefringence in certain polarization directions, it has been proposed, for example, to dispose the crystal lattice of the fluorite crystals in certain orientations with respect to one another. Details relating thereto are to be found in WO 02/099500 A2, US 2003/0011896 A1 and WO 02/093209 A2. Complete compensation for disturbances in the polarization distribution caused by intrinsic birefringence is, however, generally not possible by means of these measures.
Another approach to preventing intensity fluctuations that occur upstream of a polarizing element as a result of disturbances in the polarization distribution has been disclosed in U.S. Pat. No. 6,252,712. A correction device compensates for disturbances in the polarization distribution. To this end, the correction device has a plate that is introduced into the beam path of the projection lens. The plate is made of magnesium fluoride and is thus birefringent. The thickness of the plate varies locally, which results in a position-dependent compensation effect. Said known correction device is consequently suitable, for example, for compensating for residual disturbances in the polarization distribution that continue to exist despite favorable orientation of the crystal lattice of birefringent crystals.
In order to be able to compensate for as general a class as possible of polarization disturbances, the use of two birefringent plates whose major axes are rotated through 45° with respect to one another is furthermore proposed therein. Since the thickness fluctuations affect not only the polarization, but to an even greater extent the wave-front pattern of the light passing through, a quartz plate for wave-front compensation is provided for each correction plate. The quartz plates have, in turn, thickness fluctuations that vary, however, in a complementary way to those of the correction plates. However, it is precisely these additionally necessary measures that render the correction device disclosed in DE 198 07 120 A1 relatively expensive.